From a functional standpoint on decel the PCM would turn fuel learn OFF and reduce or turn off injector pulse width to conserve fuel.
No need to be commanding fuel when the engine is in decel right so if you think about it there still is air flowing but fuel is shut down so end result is decel cells should be showing lean

Its a don't care since fuel learn is off thus those cells averages are not added to LTFT total.
When back on gas pedal, tip-in, PCM would be in another cell so a total decel, cell 21 would have no effect on on gas pedal cell

In some cars with lots of changes I have seen cell 21 go as much as 25 % lean but it does not trigger a lean DTC because again decel cells are not factored into cells when fuel learn is on